In a tidal stream project the selection of the most appropriate device is of major importance. The aim of this work is to investigate the difference between two tidal farms, one with floating TSTs (Tidal Stream Turbines), the other with bottom-fixed TSTs, in terms of annual performance and its monthly variability. This investigation is carried out considering real operational conditions in a case study: Ria de Ortigueira (NW Spain), a drowned river valleys which is one of the most promising sites for tidal stream energy exploitation in the Iberian Peninsula. A 3D, high-resolution, numerical model is applied to simulate the hydrodynamics of the ria during an entire year with either the floating or bottom-fixed TSTs, and, on these grounds, determine the most representative performance parameters. Significant differences emerge in the performance of both plants; these are due to a great extent to the vertical variation in the flow velocity, which is relevant at many sites of interest for tidal stream energy exploitation such as Ria de Ortigueira. Finally, relevant variations were identified in the intra-annual performance which must be borne in mind in dimensioning the plant.